Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Bion® device (Bion® is a registered trademark of Advanced Bionics Corporation, of Valencia, Calif.), such as that disclosed in U.S. Published Patent Applications 2005/0021108, published Jan. 27, 2005, 2005/0057905, published Mar. 17, 2005, and 2004/0059392, published Mar. 25, 2004, which are all incorporated herein by reference in their entireties. However, the present invention also has applicability in other implantable stimulator devices, such as Spinal Cord Stimulation (SCS) devices, an example of which can be found in U.S. Pat. No. 6,516,227, which is incorporated herein by reference in its entirety
Bion® devices are typically characterized by a small, cylindrical housing that contains electronic circuitry that produces the desired electric stimulation current between spaced electrodes. These devices, also referred to as microstimulators, are implanted proximate to the target tissue so that the stimulation current produced by the electrodes stimulates the target tissue to reduce symptoms or otherwise provide therapy for a wide variety of conditions and disorders. Further examples of microstimulators can be found in the following disclosures, which are all incorporated by reference in their entireties: U.S. Pat. Nos. 5,312,439; 5,193,539; 5,193,540; 5,405,367; 6,185,452; 6,315,721; 6,061,596; 6,051,017; 6,175,764; 6,181,965; 6,185,455; 6,214,032; 6,164,284; 6,208,894.
As is known, and as shown in FIG. 1, an implantable stimulator device 10 generally comprises a battery 12. The battery 12, which may or may not be rechargeable, generally provides power to the internal logic circuitry of the device 10 (not shown). Importantly, the battery 12 also provides the power necessary to send a therapeutic stimulating current through the stimulating electrode(s) 14 on the device, so that such current is directed to tissue 16 to be treated. In this regard, the device 10 is generally programmed to provide a constant current pulse (Iout) to the tissue 16. The current is ultimately provided by a current source 20 coupled to the electrode 14, and because such a current source is generally digitally programmable to provide a precise amount of current, such a current source is generally referred to as a Digital-to-Analog Converter, or “DAC” 20. Ultimately, the current provided to the tissue 16 is set with respect to a reference potential (usually ground) as designated generically by node 14′, which may comprise another electrode on the device 10 (and which may or may not be coupled to a DAC of its own), the case of the device 10, or some other potential.
For the DAC 20 to be able to provide the programmed output current, Iout, the DAC 20 must receive a power supply voltage, which is typically called the compliance voltage, V+, and which is generated via a DC-to-DC converter circuit 22. This generated compliance voltage V+ is variable, and its optimal value, V+(opt), depends on the magnitude of the programmed stimulation current, the resistance of the tissue 16, and other factors. Determination of the optimal compliance voltage V+(opt) is accomplished by a compliance voltage monitoring and adjust circuit 18. The details of compliance voltage monitoring and adjust circuit 18 are not directly germane to this disclosure, but in any event further details concerning such circuitry in an implantable stimulator device 10 can be found in the following documents published at www.ip.com, which are both incorporated herein by reference: #IPCOM000016848D, published Jul. 18, 2003, #IPCOM000007552D, published Apr. 4, 2002. Basically, if V+ is too low, the DAC 20 will become “loaded” and unable to provide the desired current, Iout. If V+ is too high, the DAC 20 will be able to provide the desired current, Iout, but power will be wasted: i.e., some portion of the compliance voltage V+ will be dropped across the DAC 20 without any useful effect. Therefore, the basic purpose of compliance voltage monitoring and adjust circuit 18 is to deduce V+(opt) so that V+ is not to high or too low.
In any event, DC-to-DC converter circuitry 22, which is the focus of this disclosure, receives this deduced optimal compliance voltage V+(opt), and attempts to match the actual compliance voltage V+ it produces to that value. The basic function of the converter circuitry 22 is to boost the voltage, Vbat, provided by the battery 12 in the device 10 to produce a higher compliance voltage V+ matching V+(opt). To provide some exemplary numbers, Vbat might constitute between 3 to 5 Volts, from which the converter circuitry 22 generates a compliance voltage V+ of up to 18 Volts.
In the prior art, DC-to-DC converter circuitry 22 generally comprises two different types of circuits: step-up converters and charge pumps. As is known, a step-up converter utilizes a switched LC circuit to charge a coil, and then forces the charge on the coil to a capacitor. (The operation of a step-up converter will be explained in more detail later). Such step-up converters can achieve high voltages required for the compliance voltage in an implantable stimulator device application, but suffer from drawbacks. First, the efficiency of step-up converters becomes increasingly worse as higher compliance voltages are called for. Specifically, discharge of the capacitor on the switch is particularly lossy when high voltages are being generated. Such power loss at high voltages is of particular concern in implantable stimulator devices, because, as noted, such devices are normally powered by batteries 12; if power is needlessly wasted, the device 10 could fail sooner, or might require more frequent recharging, thus inconveniencing the patient in which the device is implanted.
Another drawback to the use of step-up converters involves the necessity of the charging coil. Because implantable stimulator devices are generally small, and space within the device is at a premium, it is generally advantageous to use the coil in the step-up converter for other purposes, such as for receiving EM/RF energy for externally charging the battery or for receiving and sending data telemetry between the device and an external components. However, if a single coil is used for these functions and as the coil in the step-up converter, the step up converter may place high voltages across the coil. Other circuits 30 also using the coil for other functions may have difficulty handling such high voltages, and would need to be selectively isolated or risk being damaged. In short, use of a step-up converter for converter 22 has limitations in an implantable stimulator device application, particularly when high compliance voltages are required.
Another circuit useable as a DC-to-DC converter 22 in an implantable stimulator device 10 is a charge pump. As is known, and as will be explained in further detail later this disclosure, a charge pump employs a capacitor whose top and bottom plates are selectively charged in an alternating fashion by a series of clocking signals, with the effect of boosting the input voltage to a higher DC value. Charge pumps are generally very efficient when compared to step-up converters. However, it is generally more difficult to tailor the output voltage of a charge pump to a precise value, which hampers its utility in an implantable stimulator device application, which requires sensitive and precise compliance voltage adjustment. To address this implementation drawback, a large number of additional capacitors could be used in conjunction with the charge pump, but such additional capacitors would likely need to be “off chip,” i.e., off the integrated circuit on which the current sources and other logic circuitry are formed. Using additional off-chip capacitors adds to the complexity of the device, and requires additional space, which as just noted is a significant constraint in an implantable device.
Accordingly, the implantable stimulator art would benefit from improved DC-to-DC converter circuitry for adjustably boosting the battery voltage to the compliance voltage needed to provide power to the stimulating electrode(s). Embodiments of such a solution are provided herein.